Communications in the range of frequencies currently used for wireless communications are approaching the limit. To solve this problem, the communication technology that adds the orbital angular momentum (OAM) to wireless signals to form signals into spiral beams and transmits and receives them is under study. The signals which are formed into spiral beams have the characteristics that the equiphase surface is twisted in a spiral form. By changing the spiral pitch of the equiphase surface of spiral beams, it is possible to form infinite orthogonal mode signals. Therefore, by use of spiral beams for wireless communications, it is possible to perform multiple communications over the same frequency, thereby achieving higher-speed and higher-capacity communications.
The study on communications using signals in spiral beams to which the orbital angular momentum is added is disclosed in Non Patent Literature 1 and Non Patent Literature 2, for example. Non Patent Literature 1 discloses formation of a spiral beam where the orbital angular momentum is added to a signal by use of an OAM antenna 50 having a reflecting unit 51 where a cut is made in a part of a parabolic antenna as shown in FIG. 16. Non Patent Literature 2 discloses the experiment showing that, as a result of forming a spiral beam using the OAM antenna 50 and transmitting a signal, this signal can be received at a receiving point located at a long distance.